1. Field of the Invention
This invention relates to scanning systems for microarrays of biological species such as nucleic acids or proteins, and for any type of procedure or analysis where very rapid illumination, observation, and/or detection is performed at a large number of individual sites arranged in a regular two-dimensional array. In particular, this invention relates to moving coil actuators as a driving mechanism for reciprocating motion of an optical system.
2. Description of the Prior Art
Microarrays are two-dimensional arrays of sites that are often of microscopic dimensions, with a different molecular species adhered to each site. These arrays are placed on glass slides, microtiter plates, membranes, and other two-dimensional supports, and one of their widest uses is in binding assays for the identification or characterization of an unknown biological species or the analysis of a sample for its inclusion of species that have certain binding affinities. The placement of a microarray on a support is typically done with sophisticated printing procedures and can be done on a very large scale. A single glass slide can contain a microarray of 10,000 genes, for example. Microarrays are extremely useful in the performance of multiplex experiments at high speed to obtain both qualitative and quantitive results. In a typical binding assay performed on a microarray, the individual spots of the array contain different DNA fragments, and the entire array is placed in contact with a sample containing an unknown DNA or other DNA-binding species that has been labeled to emit a luminescent signal when exposed to excitation light. Excitation and detection are then performed by way of an optical system that scans the microarray by traveling across individual rows of spots in succession, or by moving the microarray relative to the optical system with the same result. A laser-equipped scanning head is typically used for excitation.
One of the methods for producing the back-and-forth motion of a scanning head across successive rows in a two-dimensional array is by the use of a moving coil actuator. Moving coil actuators utilize the well-known Lorentz force to offer high speed movement and a high degree of control and variability, and can be manufactured to meet precise specifications. The moving coil actuators that are commonly used for scanning heads are voice coil actuators, which are direct-drive, limited-motion devices that utilize a permanent magnetic field and an electric coil to produce a force proportional to the current applied to the coil. Examples of voice coil actuators in current use include various products from the Kimco Magnetics Divison of BEI Technologies, Inc., San Marcos, Calif., USA, and from H2W Technologies, Inc., Valencia, Calif., USA. Disclosures of voice coil actuators are found in U.S. Pat. Nos. 6,894,408, 6,870,285, 6,815,846, and 6,787,943.
A moving coil actuator derives its effectiveness in part from a high force-to-mass ratio, which produces high acceleration of both the actuator and a payload. In scanners, the payload is typically a mirror and lens assembly plus any necessary holders or bearings. The force-to-mass ratio is proportional to the product of the magnetic field across the conductor and the current density in the conductor, divided by the mass density of the coil and payload. The peak current density is limited by thermal considerations arising from heating of the coil, since electrical resistivity in the coil rises with increasing temperature, and increases in the current density cause the temperature to rise. Heat accumulation is thus detrimental to the efficiency of the actuator. Heat can also cause dimensional distortion of the attached payload, and when the payload contains optical components, this can destroy optical alignment. Another factor affecting the efficiency of a moving coil actuator is the size and strength of the magnet. The cost of the magnet is directly proportional to the desired field strength and to the gap width.
Because of the movement of the coil relative to the magnetic poles, the force constant of the actuator, i.e., the motor force per unit of input current, varies with the position of the coil along its length of travel. In the simplest voice coil actuators where the coil and magnetic field are coextensive, the coil is only fully within the magnetic field when the coil is at the center of its travel. The force constant is thus at its peak in this position and tapers off toward the two ends of the travel. The greatest force is needed at the ends of the travel, however, since it is at the ends that the inertial forces must be overcome in order to reverse the direction of travel. To achieve this result, the tapering off of the force can be eliminated either by using a coil that is longer than the magnet or vice versa, and limiting the length of travel so that a constant length of coil remains in the magnetic field. This gives rise to two configurations, one of which is termed “underhung” and the other “overhung.” In the “underhung” configuration, the magnetic poles extend beyond the coil length, allowing the coil to travel the full length of the poles without loss of the influence of the magnetic flux on the coil. In the “overhung” configuration, the coil extends beyond the magnetic poles, and the range of movement of the coil extends from one extreme in which one end of the coil is aligned with the poles to the other extreme in which the other end of the coil is aligned with the poles, with different portions of the coil, although all of the same length, lying within the magnetic field at different points along the length of travel. The underhung and overhung configurations are also means of extending the length of travel of the coil, i.e., the stroke.
While the underhung and overhung configurations achieve these goals, each has its limitations. The underhung configuration requires a relatively large amount of magnetic material, which is a major component of the cost of the actuator. In addition, the excess magnet length reduces heat dissipation from the coil causing a rising temperature which contributes to the increase in resistance as electric current continues to travel through the coil. The overhung configuration requires the actuator to move a relatively high mass of coil, thereby requiring excess force to achieve the same range of movement. In addition, the added coil length presents greater resistance to the electric current, thereby requiring a higher voltage and causing more resistance heating to occur within the coil. Furthermore, both the underhung and overhung designs produce a force constant that is substantially constant along the length of travel, without additional force at the ends of the stroke where greater force is needed to reverse the direction of the coil.
One of the difficulties with scanners in general is that the greatest driving force is needed at the ends of the stroke, i.e., the extreme ends of the linear scanner travel path, where the direction of the scanner is reversed. The force requirements at the ends of the stroke are determined primarily by the need to overcome inertia rather than to overcome viscosity. The payload does not vary with time, however, nor does the desired velocity pattern. Accordingly, while conventional moving coil actuators are linear, i.e., the force that they generate is proportional to the current applied to the coil, this linearity is neither essential nor desirable.